The objective is the direct bacteriologic cultivation of mycobacterium leprae, agent of human leprosy. Since their discovery, both M. leprae and Mycobacterium lepraemurium (agent of murine leprosy and interim model for M. leprae) have been regarded as obligate intracellular microbes. In 1972 Nakamura described a semisynthetic system in which M. Lepraemurium could accomplish a single cycle of growth at 30 degrees. We have developed ultrasensitive determinations of ATP (adenosine triphosphate) in order to obtain precise information on the physiologic status and growth of host dependent agents. Continuous growth of M. lepraemurium at 38 degrees (host temperature) has been obtained by using ATP data to optimize, then to further supplement the Nakamura system. The concepts and conditions defined provide a scientific basis for undertaking the cultivation of M. leprae.